I've been playing around with Aqua Data Studio (http://www.aquafold.com). It's Java based and handles the major databases. They just updated to version 6 but made the prior versions free for personal and academic uses.

I'm using the previous major 4.7 version to do SQL (by hand and thru its Visual Query Builder) to figure out the mySQL database for an electronic medical record system.

In the past I've used TOAD tools (http://www.toadsoft.com) for Oracle databases. But now I see they have different modules for the other major database systems.